There is known an exhaust gas purifying apparatus which includes a reducing catalyst for reducing and purifying nitrogen oxide by a reducing agent, and an injection nozzle for injecting and supplying the reducing agent into an exhaust pipe upstream of the reducing catalyst, and determines that a constituent of the reducing agent are accumulated in the exhaust pipe when the difference between pressures of exhaust gas upstream section of the exhaust pipe to which the reducing agent adheres and a pressure of exhaust downstream of this portion becomes equal to or higher than a predetermined pressure, it is (see patent publication JP-A-2005-273549). Furthermore, patent publications JP-A-2005-188396, JP-A-2005-214175, JP-A-2003-269147, JP-A-6-2530 are related to the present invention as prior art.
In the patent publication JP-A-2005-273549, accumulation of the constituents of the reducing agent in the exhaust pipe is determined by referring to the pressure difference between the front side and the rear side of the section in which the reducing agent is adhered to. However, the pressure in the exhaust passage also varies when the reducing agent arrives at the exhaust gas purifying catalyst or the particulate filter. The pressure in the exhaust passage varies in accordance with an amount of the reducing agent added into the exhaust passage. Therefore, it is conceived that the pressure varies differently between when anomaly exists in the reducing agent addition device and when the reducing agent addition device is in a normal state. Thus, the reducing agent addition device such as the injection nozzle can be diagnosed by referring to the pressure variation in the exhaust passage. However, the above-described patent publications do not disclose any diagnosis of reducing agent addition device based on a pressure in the exhaust passage.
Hence, it is an object of the present invention to provide an exhaust gas purifying apparatus for an internal combustion engine capable of diagnosing a reducing agent addition device based on a pressure in the exhaust passage when a reducing agent is added to exhaust gas.